feat: Support multiple HTTP headers with the same name

[racpast]

This PR adds support for specifying multiple HTTP headers with the same name for DoH queries, which is a standard feature of HTTP.

Problem:
Currently, if a user provides multiple --http-header flags with the same name (e.g., for cookies or other custom use cases), only the last one is used. This is because the headers are stored in a map[string]string, which overwrites previous values for the same key.

Solution:
I've made the following changes:

1. Modified resolver.go to parse headers into a map[string][]string.
2. Updated the transport.HTTP struct in transport/http.go to accept map[string][]string.
3. Changed the request logic in transport/http.go to loop through all values for a given header name and use req.Header.Add() instead of req.Header.Set().

---

Verification

I have verified the fix using tshark to inspect the raw network traffic.

1. Start tshark to capture traffic:

tshark -i 4 -f "host 101.102.103.104" -o tls.keylog_file:"C:\temp\sslkeys.log" -Y http2 -V

2. Run the modified q with duplicate custom headers:

set SSLKEYLOGFILE=C:\temp\sslkeys.log
q.exe -s https://101.102.103.104/dns-query -t A google.com --http2 -i --http-header "X-Test: value1" --http-header "X-Test: value2"

3. tshark Output Confirmation: The captured HEADERS frame clearly shows that two separate X-Test headers were sent, confirming the fix is working correctly at the protocol level.

        Header: x-test: value1
            Name Length: 6
            Name: x-test
            Value Length: 6
            Value: value1
            [Unescaped: value1]
            Representation: Literal Header Field with Incremental Indexing - New Name
        Header: x-test: value2
            Name Length: 6
            Name: x-test
            Value Length: 6
            Value: value2
            [Unescaped: value2]
            Representation: Literal Header Field with Incremental Indexing - Indexed Name
            Index: 62